messageTray: Add source policy setter

Commit 8f15193b4 changed the `policy` property from a regular JS property to
a getter. This was necessary to avoid calling an overridden _createPolicy()
method before a subclass is properly initialized, but it broke the second
way of using notification sources:

Don't create a Source subclass, but use the base class directly and change
its `policy` property.

There's no good reason why we should no longer allow this, so add a setter.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/431

js/ui/messageTray.js

@@ -708,6 +708,12 @@ var Source = class Source {
         return this._policy;
     }
 
+    set policy(policy) {
+        if (this._policy)
+            this._policy.destroy();
+        this._policy = policy;
+    }
+
     get count() {
         return this.notifications.length;
     }